Department of Cell Biology Faculty

Janice L. Brissette, Ph.D.

Research Summary

My laboratory is interested in the development and diseases of the skin. Within these broad subjects, we focus on the control of epithelial morphogenesis and seek a better understanding of how the skin: 1) creates and regenerates epithelial tissues, 2) directs different cell types to develop and function as units, 3) terminates tissue development, and 4) protects itself from uncontrolled cell growth. To gain insight into these processes, we are determining the molecular and cellular functions of two transcription factors -- Foxn1 (forkhead box n1) and Hr (hairless). In humans and mice, these factors are critical to skin morphogenesis, as Foxn1 promotes tissue development, while Hr arrests development and promotes quiescence. These factors thus play fundamental but distinct roles in the health of the skin and represent gateways through which to uncover global mechanisms of morphogenetic control.

Foxn1 and the Development of Complex Traits

Foxn1 (Whn, Hfh11) is a transcription factor containing a winged-helix DNA-binding domain and a negatively charged transactivation domain. In mice, the loss of Foxn1 function results in the nude phenotype, which is characterized by defects in the epidermis, hair coat, thymus, mammary gland, nails, and pattern of pigmentation. Human FOXN1 is 86% identical to its murine counterpart, and the loss of FOXN1 function is associated with T cell immunodeficiency, congenital alopecia, and nail dystrophy, a disease distinguished by the absence of hair and mature T lymphocytes. Thus, FOXN1 deficiency produces pathologies similar to the nude phenotype, demonstrating the functional conservation of the Foxn1 orthologs.

Based on our studies of animals and cultured cells, we have developed the following model of Foxn1 function in the skin. In the epidermis and hair follicles, epithelial cells activate Foxn1 as they lose the ability to multiply and initiate terminal differentiation. In a site-dependent manner, Foxn1 then promotes up to three developmental processes. The first process is epithelial differentiation. In the Foxn1-expressing cell, Foxn1 stimulates early features of terminal differentiation and suppresses late features, enabling the cell to proceed through its differentiation program in proper sequence. The second process is epithelial growth or self-renewal. As a cell begins its differentiation program, Foxn1 induces the cell to release mitogens, which then stimulate nearby epithelial cells to divide. The differentiating cell thus drives the production of new cells, increasing the size of the epithelium or replacing transient cell types. The third process is epithelial melanization. Through intercellular signaling, Foxn1 identifies an epithelial cell as a target for pigmentation, attracts melanocytes to the cell, and ultimately stimulates pigment transfer. Thus, according to this model, Foxn1 plays an unusual role in epithelial development, as the protein induces and integrates growth, differentiation, and pigmentation, uniting these processes by orchestrating the actions of distinct cell populations.

Our studies of Foxn1 further suggest a new view of melanized epithelial cells and their role in pigmentary system development. As Foxn1-positive cells instruct melanocytes to melanize them, Foxn1 confers distinct properties on epithelial cells, which we call a "pigment recipient phenotype." A cell with this phenotype recruits pigment donors (melanocytes) and subsequently engineers its own pigmentation. As such, the pigment recipients constitute a specialized, epithelial counterpart to the melanocytes. As recipient cells determine where pigment is placed, the pattern of recipients ultimately determines the pattern of pigmentation. In essence, recipients render the skin analogous to the coloring books of children. As recipient cells develop, they collectively form a "picture" -- a blueprint for pigmentation -- that is colorless initially but outlined by the recipients themselves. Melanocytes then melanize the recipients and "color in" the picture. Thus, the recipients provide a template that melanocytes follow as they pigment the skin.

To coordinate multiple processes, Foxn1 must cause differentiating cells to emit signals, which then regulate the behavior of melanocytes and other epithelial cells. We have identified one of these signals, as we have shown that Foxn1 induces the synthesis and secretion of Fgf2. To activate Fgf2 expression, Foxn1 binds to the Fgf2 promoter, making Fgf2 the first example of a Foxn1 target gene. Fgf2 is known to have potent effects on melanocytes, as it can induce their proliferation, differentiation, survival, melanogenic activity, or chemotaxis. Moreover, Fgf2 is a mitogen for keratinocytes and can stimulate proliferation in a paracrine manner. Thus, by inducing the secretion of Fgf2, Foxn1 promotes two processes at once -- epithelial growth and epithelial pigmentation. Recently, we have identified additional signals that are induced by Foxn1 and appear to regulate the development of epithelial cells and melanocytes. Our current aim is to elucidate the roles of these Foxn1-activated signaling systems and the mechanisms by which these systems operate.

Like the skin, the thymus and mammary gland require Foxn1 for the proper proliferation and differentiation of their epithelial cells. In the absence of Foxn1, the primordial thymic epithelium fails to grow and differentiate into a mature network. Likewise, in nude mice, the mammary epithelium exhibits a generalized impairment of its growth, differentiation, and branching. As such, Foxn1 appears to play similar roles in different organs, generally driving epithelial growth and differentiation forward. To play these similar roles, Foxn1 presumably activates similar pathways and controls similar sets of effectors (e.g., intercellular signals). Moreover, as the thymus develops, the epithelial cells, which are Foxn1 positive, are surrounded by mesenchymal cells, which like melanocytes arise from the neural crest. Accordingly, we think it likely that Foxn1 enables thymic epithelium to interact with the mesenchyme, similar to the way it enables cutaneous epithelium to interact with melanocytes. Finally, it is known that the inactivation of human FOXN1 causes one disease -- T cell immunodeficiency, congenital alopecia, and nail dystrophy -- the pathology of which resembles the nude phenotype. Potentially, FOXN1 affects other disorders as well, given its effects on multiple processes. For example, abnormalities in FOXN1 may contribute to abnormalities in pigmentation (e.g., vitiligo) and abnormalities in growth/differentiation control (e.g., psoriasis). Under normal conditions, FOXN1 may protect the skin against cancer (e.g., carcinomas and melanoma), as pigmentation protects the skin against ultraviolet light, the principal cause of cutaneous malignancies. Moreover, the FOXN1 pathway may provide insight into cancer development, as cancerous cells (e.g., melanoma cells) typically free themselves of the need for support (growth and survival factors) from the surrounding tissue, and under normal conditions, FOXN1 may furnish that support. Thus, the identification of FOXN1-induced signals may facilitate the identification of critical changes in cancer cells, which allow them to achieve independence from their environment. To test this idea, we are currently determining whether melanoma cells produce for themselves the signals normally provided by the FOXN1-positive epithelial cells. In all, the Foxn1 pathway activates and integrates the morphogenesis of multiple cell populations. These actions should provide a model for how different cell types work together to generate complex traits.

Hr and the Termination of Tissue Morphogenesis

The Hr (hairless) protein is essential for the skin's health, but its role in skin cells is largely unknown. In mice lacking Hr, the skin develops normally and produces a full coat of hair. But upon completing the first round of hair morphogenesis, the hair follicles degenerate rapidly en masse, resulting in total, permanent baldness. Concomitantly, the epidermis thickens, and the skin develops an increased susceptibility to epithelial cancers. In humans, HR is conserved in sequence and function, as the human protein is 79% identical to its murine counterpart and mutations in human HR cause congenital atrichias, disorders closely resembling the Hr-mutant phenotypes of mice. Thus, in mammalian skin, Hr becomes necessary when epithelial development ends, as it enables the hair follicles to shift from hair production to quiescence, the epidermis to keep its proper size, and ultimately, the skin to protect itself from cancer.

Our studies of Hr have produced the following preliminary model of Hr function. When the skin and hair are fully formed, Hr promotes the termination of tissue morphogenesis and does so by reducing the number or activity of epithelial progenitor cells. Hr restricts progenitor cell populations by either inhibiting their growth or promoting their death, the latter accomplished primarily via apoptosis but also perhaps via terminal differentiation. Hr performs these functions through its interactions with phosphatases and histone demethylases. Thus, Hr serves as an activator of the final (and perhaps least studied) stage of tissue development, namely, the bringing of morphogenesis to a close. Our future studies will test this model and delineate the processes and genes under Hr control.

In summary, we are elucidating two regulatory systems that perform essentially opposite functions. One system, represented by Foxn1, induces and organizes the development of new tissue; the other system, represented by Hr, terminates development. Together, these systems should serve as gateways through which to understand basic mechanisms of morphogenesis and disease.